Posts Tagged ‘electronic art’

Patterns caused by different frequencies

21 January 2018

interference pattern
The above photo (which I have colorized and cropped) from Wikimedia Commons illustrates how two similar wave patterns can interfere with each other.

I have been working with this basic phenomenon as a possible way to create interesting patterns in LED displays that could be configured to interact with the environment.

My simpler prototype uses 12 yellow LEDs in a circle. The illustrations below show them in rows. This made it much easier to draw the illustrations.

Two Signals

The basic idea is to compare two different signals in a way that is interesting.

This design uses voltage-controlled oscillators to create a pair of square waves.

Each wave is then applied to a circuit that turns it into a repeating pattern with twelve parts. The corresponding outputs are then compared, and an LED is turned on or off depending on the result of that comparison.

The comparison logic I used for my first prototype turns the LEDs on only when both outputs being compared are “on” (about +5 volts in this case). There are other types of logic possible. This particular one gives the lowest current usage from the power supply.

So there are 12 LEDs, and each LED can only be on for one-twelfth of the time it takes for the pattern to repeat. But they will all fully light during their time slot only if the two signals being compared are exactly the same.

Otherwise, the LEDs will turn on and off in a pattern based on how different the two signals are. Here I have illustrated a few possible patterns. The yellow strip is the “reference” frequency, and the light blue strip is the other frequency. The lime green bands depict which LEDs will turn on, and for how long, based on my chosen comparison logic.

If the two signals are close but not exactly the same, the circle of LEDs will dim and brighten as the signals slowly go in and out of alignment. This is similar to how two musical notes that are close to each other will “beat” (get louder and softer).

two similar frequencies

If the two signals are a lot different, but related mathematically, they will produce a pattern of light and dark in the display.

As it is almost impossible to adjust the two oscillators to exact frequency multiples, the actual result is a fast or slow rotation of the pattern, depending on how far off they are.

Oddly, a 3 times difference in frequency produces a 2-node pattern.

frequencies different by factor of three

And a 4 times difference in frequency produces a 3-node pattern.

frequencies different by factor of four

Here is an image of my prototype – doctored to remove most of the distracting details of wiring and so forth on the circuit boards – showing a 3-node pattern that is slowly rotating. Here, three LEDs are much brighter, and three others are just beginning to turn on as the pattern slowly rotates.

3 nodes in circle

Three node pattern with LEDs in a circle.

Other Designs

I also made a display that uses my “signature” pattern of 8 concentric circles, starting with one having only three LEDs in it, and ending with one that has ten LEDs. This is a more interesting display to watch, but the results are more difficult to interpret. It is also more difficult to make. So I will likely continue to work with simpler designs as I develop this idea.

Software

These designs don’t use any software; totally hard-wired, as they say. My experience with software that runs on controllers that I can afford is that it doesn’t run fast enough to provide a smooth display that doesn’t blink. So, though I plan to use controllers in some of these designs, I prefer designs where they are not needed and we are dealing totally with the real-time interactions of signals.

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In search of a better SerDes

17 October 2017

Oh no! Another dry technical article! True, true. Just pass it by if you’re not interested.

Serdes sounds like a Greek word, but it isn’t really. There are some people with the name Serdes, but it is uncommon. I learned it as an engineering acronym thus (lightly edited):

A Serializer/Deserializer (SerDes) – usually pronounced “sir-deez” – is a pair of functional blocks commonly used in high speed communications to compensate for limited input/output. These blocks convert data between serial data and parallel interfaces in each direction. The term “SerDes” generically refers to interfaces used in various technologies and applications. The primary use of a SerDes is to provide data transmission over a single/differential line in order to minimize the number of I/O pins and interconnects.

https://en.wikipedia.org/wiki/SerDes

Electronic Art

I am trying to work out some cool things to do with LED arrays that would respond to environmental or observer inputs. There are many pieces to such a system. This includes the possibility that the display itself may be some distance from the electronics that collects the input signals and decides what the display should do. The same way a computer monitor can be separate from the computer. And in this case, the two are connected with a cable.

Some of us remember the old printer cables. They were thick, had up to 25 separate wires in them, and couldn’t be much longer than 15 feet. I could probably use such a cable in my projects. But that’s a lot of bulk, and it comes with many limitations. Those cables connected a parallel port on the computer to a parallel port on the printer. There were usually 8 data lines plus a bunch of handshaking signals to make it so the computer would not send data faster than the printer could print it out. If you wanted to get a lot of data back from the printer, you could add another 8 data lines going in the opposite direction.

8 bits can encode into 256 different numbers (0 to 255). That’s enough for an alphabet – both upper and lower case – and a bunch of other symbols. Each symbol has a number “code” that stands for that symbol. Both ends of the line have to use the same code system.

An 8 bit parallel system could go pretty fast; millions of symbols per second. But try pushing parallel data through a long cable that fast and you will quickly run into problems. You would need to shield the cable so external signals won’t interfere with it, and so it won’t radiate signals into external equipment. And the wires in the cable, when they get quite long, resist fast signals going through them in at least three different ways (resistance, inductance and capacitance). So if you want to send data fast through a long cable, you have a whole hardware design challenge on your hands.

SerDes concept

Illustration of the SerDes concept. Original graphic by GrĂ©goire Surrel – Own work, CC BY-SA 4.0

The solutions to these problems usually involve reducing the number of wires carrying signals (ideally just one pair would be enough) and creating special hardware interfaces that alter the signals so that they will make it through the cable successfully, even though the cable presents various barriers to proper transmission.

A standard solution for many years was the “RS-232” serial cable. In this system the signal is amplified to make it more resistant to interference and cable attenuation. And the signal is “serialized” so it only has to use one pair of wires. That means each symbol of 7 or 8 bits would be transmitted as a sequence of bits that would have to be reassembled into 7 or 8 parallel bits at the receiving end. That was an early SerDes system. But we didn’t call it that in the old days. The acronym only came into wide use after the internet and its various forms of information exchange came into wide use. The term commonly refers to high speed data transmissions, but the basic concepts are the same regardless of data rate. My projects use quite low data rates just to make sure I don’t run into too many design problems and can use cheap parts.

The RS-232 standard could probably work for me, but I wanted to try another more modern data transmission standard, TIA-485. (RS = Recommended Standard, as published by the EIA, Electronics Industries Alliance, but now taken over by the TIA, Telecommunications Industry Association). This standard uses two wires for each signal plus a third wire used as a ground (zero volts) reference. The signal is transmitted in an attenuated form, differentially. That means a “zero” would be transmitted by putting maybe 3-1/2 volts on one wire and 1 volt on the other. And a “one” would be transmitted by reversing those. Smaller signals in a cable create less external interference and are easier to pass through longer cable lengths.

I have a connector that is used for MIDI (Musical Instrument Digital Interface) that has five pins, which means it can carry two differential serial signals (or 4 RS-232 signals) plus a ground reference wire. I wanted to use this connector and a 5-wire cable, but there was one problem:

SerDes Timing

Just as in the old parallel printer cables, where handshaking signals were necessary to tell the printer when a symbol to print was put onto the connecting cable, and tell the computer when the printer was busy, serial systems also need a way of at least telling the receiver when the transmitted data is good, how fast it is going, and when an entire symbol has been sent. This requires, minimum, clock and end-of-symbol signals for data rate and data synchronization. In the RS-232 system, the data rate had to be set at both ends in advance. And the end-of-symbol signal was coded into the data stream. It takes a computer to figure out how to decode this data stream, but if you send all three signals separately, you don’t need any computing at the receiving end. Deserialization can be done with one piece of hardware called a shift register.

But I can’t transmit three signals over a five-wire TIA-485 cable, only two. So I thought I’d figure out how to combine the three signals into two so that my system could work with the hardware I have. I devised a rather simple system to do this, and built an initial working system several years ago for my “Christmas” project (Christmas because it used strings of holiday lights for the visual display). Recently I have built two more systems that use this method.

Glitches

I like to re-design systems each time I build them. This is partly because I might not have the same parts available that I used in an earlier design. Or it might be just to explore different ways the problem could be solved. All the heavy work in my SerDes system is on the transmitting site. The receiver is very easy to make. And for this transmitter design I wanted to use counters to run ICs (integrated circuits, now often known as “chips”) called multiplexers. You put parallel data on the 8 inputs of a multiplexer, then tell it which input to put on the output using a counter. And if the counter repeats a regular pattern (as most do) then the parallel data at the inputs will come out of the output in a predictable serial sequence. And so you have achieved serialization.

In my first design I was getting “glitches” at the outputs of some of my multiplexers. This is because I was using “ripple” counters. In this type of counter, the counting outputs don’t all change at exactly the same time. They might be a little off (usually much less than a microsecond, but that’s enough time to cause trouble). In other words, when changing from count 1 to count 2 for instance, the ones bit has to change from one to zero, while the twos bit has to change from zero to one. If the twos bit lagged a little, both outputs might be zero for a split second, telling the multiplexer to go to the wrong input. Such glitches can be filtered out, which is what I did in the first design. But in the second design I decided to try a different counting scheme, where only one counting bit would change at any one time. This should make counting glitches impossible (it does). But it means the count is no longer in number sequence. In other words to do this with a 4-count pattern, you have to use the pattern 0-2-3-1 (or 0-1-3-2) to get a glitchless count. This different sequence is not a problem when using a multiplexer, though it is more confusing to design if you are used to using ripple counters that count 0-1-2-3 (etc.).

I looked at the waveforms associated with this kind of counting, and they were just two square waves offset by one count. I found a PDF online that describes how to implement this kind of counter. It’s called a “quadrature” counter, and it’s pretty simple to do. Getting a similar sequence of 8 is a little more tricky, but basically just interleaves a quadrature signal with a square wave. I built my second system this way and it works fine (though I had to scratch my head a bit to get the input sequence right, as it is sensitive to the place value given to the various counting signals).

quadrature waveform illustration

A four-count pattern implemented using a quadrature counter.

What form should the data take?

So I now have a hardware system that can be used with either 2 TIA-485 signals over quite long distances (if the cables are made well) or with 4 RS-232 signals (but not the RS-232 encoding system). The RS-232 version is much easier to build, but does have distance and speed limitations compared to the TIA-485 system.

The original intention of the system was to enable transmission of 8-bit-wide signals that would be used to control an array of LEDs. But it could also be used to transmit serial control streams of any bit length. This means a wide variety of displays could be controlled, as long as they didn’t have to change at a very fast rate. In other words, we’re not talking about full-motion video, like TV, but that’s not the sort of display I’m working with. My average display contains less than 100 LEDs, while a modern TV screen contains millions.

I have also tried transmitting analog data using digital serial techniques by using pulse-width encoding, which is very simple to implement in hardware. This gives me the option of using digital data transmission instead of long analog signal lines. This may come in handy in some of my projects.

Electronics Design Case Study – ADSR

23 September 2017

ADSR module homemade

This is a technical article and if you have no particular interest in electronics design feel free to skip it. It will get into some terminology that won’t all be explained in the text…

Music Synthesis

My interest in synthesizers goes back to my early days studying electronics. I always wanted to make my own synth.

But by 1983, MIDI had come out, and I was in the Sea Org.

MIDI stands for Musical Instrument Digital Interface. Musical instruments were an early target for embedded controllers (software-controlled electronic circuits) for many reasons. This ended (mostly) the era when synthesizers were controlled by analog (continuous) signals. Voltage sources were terrible when it came to keeping all the electronic instruments in an ensemble in tune with each other. So tuning was an obvious feature to turn over to the digital world, where crystal-controlled oscillators could stabilize pitch to within a few parts in a million.

The advent of digital signal processing meant things like voltage-controlled filters and unusual effects like ring modulation could be implemented with algorithms instead of hardware.

Voltage-controlled amplitude, however, is so straightforward in the hardware realm that it remains somewhat popular. A basic part of synthesizing a real-world note or sound is approximating its amplitude envelope. This envelope has long been analyzed by acoustics engineers into four parts: Attack; Decay; Sustain and Release. If you play a note on a stringed instrument you can easily see each of these parts in action. How hard and fast you hit (or pluck) the string determines the initial attack and to some extent how that attack decays. Then if you don’t damp the string it will continue to ring until it is damped or played over. This is sustain. And when it is damped, the sound will die out, which is the release phase.

There are many many possible ways to imitate this amplitude envelope with electronics. The most common methods use parts that I had run out of (1 Meg-ohm potentiometers) so I decided to try an alternative design of my own creation.

Design Requirements

Most traditional ADSRs take a “gate” signal from a keyboard which tells the electronics how long the key is being held down (“note on” in MIDI). My electronic art projects use sensors, not keyboards, so I couldn’t rely on a gate signal to determine how much sustain the sound would have. I also kind of wanted a circuit that could be adapted so that each part of the envelope could be controlled by a separate sensor. That means it couldn’t just use pots, like the super-simple designs do. I also wanted to try straight-line segments rather than the traditional curved segments you get using just resistors and capacitors, even though this is less “realistic” for decay slopes.

I had a front panel I was reusing from an earlier project, and originally loaded it with just three pots – all that seemed to fit – which is two less than you need to control the five main parameters of the envelope. But I thought I could skip setting a sustain level, and use one pot to control both decay and release slopes. This panel had to fit into a eurorack-style chassis I had put together earlier, with power coming in the front.

The unit was also to include the voltage-controlled amplifiers, using an IC I had never worked with before.

First Try

I decided to use an op amp integrator at the core of the design, as it would give straight-line slopes and could be dependably controlled. However, I wasn’t sure how to set up my 100K pots to imitate a wider range of resistance. I used three comparators and a couple of flip-flops to detect voltage levels and turn the various slopes on and off.

Mounted at the bottom of the front panel was a backplane board that has become standard in most of my designs. Circuit boards then plug into this backplane, which ideally handles all the interconnects. Front panel parts that could not be mounted directly on the backplane board would be wired down to the backplane using jumpers.

I made the envelope generator board first and then the VCA board. The VCA datasheet was confusing at first, but by wiring an actual circuit I was able to figure out what was going on. This VCA could accept a wide range of control voltages (0 to 30 roughly), but they were referenced to the negative voltage rail! So I needed both an amplifier and offset for my envelope, as it would go from 0 to 5V only, my standard range for control voltages. I realized at this point that I would need a sound source to test this with, and it would also be nice to monitor the envelope waveform on an analog meter. I then spent about a day creating an oscillator and a meter for these purposes.

An incomplete design with too many questions about “will it work this way?” resulted in my running out of room on my envelope module. To solve this I piggybacked an extra module onto the main one. I got some sort of envelope out of this design, but the pots worked only over an extremely narrow range of their total rotation. I had to decide whether to stick with these pots and basic design, or start over.

Second Try

I looked around at what my alternatives were. I had a nice set of six quite small 5K pots from an old piece of audio equipment. They would all fit into the panel if I drilled new holes for them. So I decided to go for it. Five of these pots went onto a new backplane board. I modified this board to hold circuitry and figured the majority of my new design would fit on this board, with the rest put on a new plug-in module on the original backplane.

Now that I was beginning to recognize that this was a challenging project, I went to a build a section and test it approach to my work. 5K pots could only yield 1:100 output variations (comparable to using a 1Meg pot in series with a 10K resistor) by using the turn-on “knee” of transistors to stretch out the transfer curve. I have used this before so didn’t bother to work it out in complete detail, or plot the curves graphically, but below gives you a graphical idea of what I’m referring to:

transistor turn-on graph

Using a curve found on the internet, note that a 0.1 change in input voltage produces a ten-fold change in output current. Extend this input range a little more and you can squeeze out a 1:100 input/output ratio, or even more.

I built my current sources and sinks using discrete transistors. This gives worse consistency and stability than using matched pairs or some specialized IC, but usable for my purposes. I built one and tested it. I gave me a range of 50 to roughly 1500 microamps. This was good enough. I put the rest on the board, then added a dual timer (LM556) and some inverters and connected it up to run continuously (astable mode). I powered up and checked with my oscilloscope. This part of the design worked fine. The timers have two comparators and a flip-flop inside each of them, so this decreased my parts count.

I thought that I could get the timers to stop after just one cycle using some sort of edge detection scheme. But it didn’t work. I was using two timers so that the attack-decay and sustain-release cycles were separate and could be put in sequence. But my difficulties in making the circuit cycle just once and stop caused me to rethink this approach.

The next day I rewired the timers for one-shot (monostable mode) operation. Now the timers could be triggered by my sensor, fire – producing the envelope waveforms – and would then stop, waiting for a new trigger. I used two control flip-flops with NAND gates (CD4093B) to lock out new trigger signals until the current envelope had finished. I really needed only one flip-flop, but the package (CD4013B) has two in it, so I used one for each of the timers.

Next I had to get all the analog levels of the envelope right. I put this circuitry on the new module board. I only needed 8 wires to connect the new envelope module to this analog module. It has six opamps and a comparator. The comparator detects when the envelope signal goes to zero, and resets the flip-flops so they can allow in another trigger signal. This circuit wasn’t working at first. What was wrong? The envelope waveforms were only going down to 0.5 volts, not to ground. I had the comparator set below this, so it was never firing. I was powering my envelope generator with only ground and +12V. The current sinks (set up as mirrors for my current sources) could only pull the load capacitors down to 1/2 a volt. I compensated for this by adding some offset to my summing opamp. I set the output to go a little below ground so the comparator would for sure fire. I had to find a missing wire on the envelope board before I got the unit totally working. It’s not perfect, but it now works as it was designed to work, and will serve it purpose in helping me develop electronic art that uses sounds.